如何解决通过ASHX处理程序支持可恢复的HTTP下载吗？
| 我们正在通过ASP.NET中的ASHX处理程序提供应用程序设置的下载。 一位客户告诉我们，他使用了一些第三方下载管理器应用程序，而我们提供文件的方式目前不支持他的下载管理器应用程序的“恢复”功能。 我的问题是： 恢复下载的基本原理是什么？是否有某些HTTP GET请求告诉我开始的偏移量？解决方法
恢复下载通常可以通过HTTP“ 0”头进行。例如，如果客户端只需要文件的第二个千字节，则它可能会发送标头“ 1”。
有关更多信息，请参见RFC的HTTP / 1.1页139。
, 感谢icktoofay让我入门，这是一个完整的示例，可以节省其他开发人员一些时间：
磁盘示例
/// <summary>
/// Writes the file stored in the filesystem to the response stream without buffering in memory,ideal for large files. Supports resumable downloads.
/// </summary>
/// <param name=\"filename\">The name of the file to write to the HTTP output.</param>
/// <param name=\"etag\">A unique identifier for the content. Required for IE9 resumable downloads,must be a strong etag which means begins and ends in a quote i.e. \"\\\"6c132-941-ad7e3080\\\"\"</param>
public static void TransmitFile(this HttpResponse response,string filename,string etag)
{
var request = HttpContext.Current.Request;
var fileInfo = new FileInfo(filename);
var responseLength = fileInfo.Exists ? fileInfo.Length : 0;
var buffer = new byte[4096];
var startIndex = 0;
//if the \"If-Match\" exists and is different to etag (or is equal to any \"*\" with no resource) then return 412 precondition failed
if (request.Headers[\"If-Match\"] == \"*\" && !fileInfo.Exists ||
request.Headers[\"If-Match\"] != null && request.Headers[\"If-Match\"] != \"*\" && request.Headers[\"If-Match\"] != etag)
{
response.StatusCode = (int)HttpStatusCode.PreconditionFailed;
response.End();
}
if (!fileInfo.Exists)
{
response.StatusCode = (int)HttpStatusCode.NotFound;
response.End();
}
if (request.Headers[\"If-None-Match\"] == etag)
{
response.StatusCode = (int)HttpStatusCode.NotModified;
response.End();
}
if (request.Headers[\"Range\"] != null && (request.Headers[\"If-Range\"] == null || request.Headers[\"IF-Range\"] == etag))
{
var match = Regex.Match(request.Headers[\"Range\"],@\"bytes=(\\d*)-(\\d*)\");
startIndex = Parse<int>(match.Groups[1].Value);
responseLength = (Parse<int?>(match.Groups[2].Value) + 1 ?? fileInfo.Length) - startIndex;
response.StatusCode = (int)HttpStatusCode.PartialContent;
response.Headers[\"Content-Range\"] = \"bytes \" + startIndex + \"-\" + (startIndex + responseLength - 1) + \"/\" + fileInfo.Length;
}
response.Headers[\"Accept-Ranges\"] = \"bytes\";
response.Headers[\"Content-Length\"] = responseLength.ToString();
response.Cache.SetCacheability(HttpCacheability.Public); //required for etag output
response.Cache.SetETag(etag); //required for IE9 resumable downloads
response.TransmitFile(filename,startIndex,responseLength);
}
public void ProcessRequest(HttpContext context)
{
var id = Parse<int>(context.Request.QueryString[\"id\"]);
var version = context.Request.QueryString[\"v\"];
var db = new DataClassesDataContext();
var filePath = db.Documents.Where(d => d.ID == id).Select(d => d.Fullpath).FirstOrDefault();
if (String.IsNullOfEmpty(filePath) || !File.Exists(filePath))
{
context.Response.StatusCode = (int)HttpStatusCode.NotFound;
context.Response.End();
}
context.Response.AddHeader(\"content-disposition\",\"filename=\" + Path.GetFileName(filePath));
context.Response.ContentType = GetMimeType(filePath);
context.Response.TransmitFile(filePath,version);
}
/// <summary>
/// Writes the file stored in the database to the response stream without buffering in memory,ideal for large files. Supports resumable downloads.
/// </summary>
/// <param name=\"retrieveBinarySql\">The sql to retrieve the binary data of the file from the database to be transmitted to the client. Parameters can be reffered to by {0} the index in the supplied parameter array.</param>
/// <param name=\"retrieveBinarySqlParameters\">The parameters used in the sql query. Specify null if no parameters are required.</param>
/// <param name=\"connectionString\">The connectring string for the sql database.</param>
/// <param name=\"contentLength\">The length of the content in bytes.</param>
/// <param name=\"etag\">A unique identifier for the content. Required for IE9 resumable downloads,must be a strong etag which means begins and ends in a quote i.e. \"\\\"6c132-941-ad7e3080\\\"\"</param>
/// <param name=\"useFilestream\">If the binary data is stored using Sql\'s Filestream feature set this to true to stream the file directly.</param>
public static void TransmitFile(this HttpResponse response,string retrieveBinarySql,object[] retrieveBinarySqlParameters,string connectionString,int contentLength,string etag,bool useFilestream)
{
var request = HttpContext.Current.Request;
var responseLength = contentLength;
var buffer = new byte[4096];
var startIndex = 0;
//if the \"If-Match\" exists and is different to etag (or is equal to any \"*\" with no resource) then return 412 precondition failed
if (request.Headers[\"If-Match\"] == \"*\" && contentLength == 0 ||
request.Headers[\"If-Match\"] != null && request.Headers[\"If-Match\"] != \"*\" && request.Headers[\"If-Match\"] != etag)
{
response.StatusCode = (int)HttpStatusCode.PreconditionFailed;
response.End();
}
if (contentLength == 0)
{
response.StatusCode = (int)HttpStatusCode.NotFound;
response.End();
}
if (request.Headers[\"If-None-Match\"] == etag)
{
response.StatusCode = (int)HttpStatusCode.NotModified;
response.End();
}
if (request.Headers[\"Range\"] != null && (request.Headers[\"If-Range\"] == null || request.Headers[\"IF-Range\"] == etag))
{
var match = Regex.Match(request.Headers[\"Range\"],@\"bytes=(\\d*)-(\\d*)\");
startIndex = Parse<int>(match.Groups[1].Value);
responseLength = (Parse<int?>(match.Groups[2].Value) + 1 ?? contentLength) - startIndex;
response.StatusCode = (int)HttpStatusCode.PartialContent;
response.Headers[\"Content-Range\"] = \"bytes \" + startIndex + \"-\" + (startIndex + responseLength - 1) + \"/\" + contentLength;
}
response.Headers[\"Accept-Ranges\"] = \"bytes\";
response.Headers[\"Content-Length\"] = responseLength.ToString();
response.Cache.SetCacheability(HttpCacheability.Public); //required for etag output
response.Cache.SetETag(etag); //required for IE9 resumable downloads
response.BufferOutput = false; //don\'t load entire data into memory (buffer) before sending
if (!useFilestream)
{
using (var connection = new SqlConnection(connectionString))
{
connection.Open();
var command = new SqlCommand(retrieveBinarySql,connection);
for (var i = 0; retrieveBinarySqlParameters != null && i < retrieveBinarySqlParameters.Length; i++)
{
command.Parameters.AddWithValue(\"p\" + i,retrieveBinarySqlParameters[i]);
command.CommandText = command.CommandText.Replace(\"{\" + i + \"}\",\"@p\" + i);
}
var reader = command.ExecuteReader(CommandBehavior.SequentialAccess);
if (!reader.Read())
{
response.StatusCode = (int)HttpStatusCode.NotFound;
response.End();
}
for (var i = startIndex; i < contentLength; i += buffer.Length)
{
var bytesRead = (int)reader.GetBytes(0,i,buffer,buffer.Length);
response.OutputStream.Write(buffer,bytesRead);
}
}
}
else
{
using (var connection = new SqlConnection(connectionString))
{
connection.Open();
var tran = connection.BeginTransaction(IsolationLevel.ReadCommitted);
var command = new SqlCommand(Regex.Replace(retrieveBinarySql,@\"select \\w+ \",v => v.Value.TrimEnd() + \".PathName(),GET_FILESTREAM_TRANSACTION_CONTEXT() \"),connection);
command.Transaction = tran;
for (var i = 0; retrieveBinarySqlParameters != null && i < retrieveBinarySqlParameters.Length; i++)
{
command.Parameters.AddWithValue(\"p\" + i,\"@p\" + i);
}
var reader = command.ExecuteReader();
if (!reader.Read())
{
response.StatusCode = (int)HttpStatusCode.NotFound;
response.End();
}
var path = reader.GetString(0);
var transactionContext = (byte[])reader.GetValue(1);
using (var fileStream = new SqlFileStream(path,transactionContext,FileAccess.Read,FileOptions.SequentialScan,0))
{
fileStream.Seek(startIndex,SeekOrigin.Begin);
int bytesRead;
do
{
bytesRead = fileStream.Read(buffer,buffer.Length);
response.OutputStream.Write(buffer,bytesRead);
}
while (bytesRead == buffer.Length);
}
tran.Commit();
}
}
}
public void ProcessRequest(HttpContext context)
{
var id = Parse<int>(context.Request.QueryString[\"id\"]);
var db = new DataClassesDataContext();
var doc = db.Documents.Where(d => d.ID == id).Select(d => new { d.Data.Length,d.Filename,d.Version }).FirstOrDefault();
if (doc == null)
{
context.Response.StatusCode = (int)HttpStatusCode.NotFound;
context.Response.End();
}
context.Response.AddHeader(\"content-disposition\",\"filename=\" + doc.Filename);
context.Response.ContentType = GetMimeType(doc.Filename);
context.Response.TransmitFile(\"select data from documents where id = {0}\",new[] { id },db.Connection.ConnectionString,doc.Length,doc.Version,false);
}
public static T Parse<T>(object value)
{
//convert value to string to allow conversion from types like float to int
//converter.IsValid only works since .NET4 but still returns invalid values for a few cases like NULL for Unit and not respecting locale for date validation
try { return (T)System.ComponentModel.TypeDescriptor.GetConverter(typeof(T)).ConvertFrom(value.ToString()); }
catch (Exception) { return default(T); }
}
public string GetMimeType(string fileName)
{
//note use version 2.0.0.0 if .NET 4 is not installed,in .NET 4.5 this method has now been made public,this method apparently stores a list of mime types which would be more complete then using registry
return (string)Assembly.Load(\"System.Web,Version=4.0.0.0,Culture=neutral,PublicKeyToken=b03f5f7f11d50a3a\")
.GetType(\"System.Web.MimeMapping\")
.GetMethod(\"GetMimeMapping\",BindingFlags.NonPublic | BindingFlags.Public | BindingFlags.Static)
.Invoke(null,new object[] { fileName });
}
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